Yet Another Docking Question - Maintaining Orders?

[NeroWolfe]

Okay, the scenario: I have 2 craft in orbit I'm trying to dock together. The first is a "rescue" lander, capable of landing on Eve, retrieving 3 Kerbals, and then returning to orbit. The second is an interplanetary rig (also carrying extra monopropellant for the lander), which is meant to act as a tug to pull the lander to Eve. I have managed to become reasonably capable at establishing/matching orbits, so that I can launch the rig, match orbit with the lander, intersect it, and zip right up to it within a couple of hundred meters no problem.

Where it all is falling apart is once I get within 100m. My lander, Ray, is in a slightly tumbling orbit. My big rig Venkman can match it up to a point, but once the two get close enough they continue to close and separate so quickly that by the time I've switched ships, set the autopilot to target, (the docking ports on both ships), and it lines up, the other ship is unaligned and either closing or falling away so rapidly that by the time I've switched ships, set the autopilot to target, and it lines up, the other ship is unaligned and either closing or falling away so rapidly that... and on and on until its a crash or a realign that starts the whole cycle over again. Ray's out of monopropellant and Venkman's still lugging the structurally awkward get-into-orbit stage so it's a bit of a hippo ballet anyways, but...

What I'm wondering is, (and apologies if this has come up before; I did a lot of searching and couldn't find a thread or answer anywhere), is there a way, when setting the autopilot on one of my two ships to 'target', to get it to keep autopiloting towards the target once I switch to ship two. If ship one would keep pointing itself towards the docking port of ship two (and Jeb is piloting ship one, the lazy sod) while I was in charge of ship two and telling it to keep pointing itself at the docking port of ship one, poor Bob and co would have already been home from Eve.

If it's do-able, how do I do it? And if it isn't do-able, it might be worth sticking into a future update maybe?

[mdkendall]

You just need the ship you are not flying to remain still (i.e not be rotating). It doesn't need to be being piloted towards you, it just needs to be not tumbling. (I guess it is technically possible to dock with a tumbling ship, but its pretty difficult).

You can achieve that by switching to the target ship, engaging SAS (just using the basic "Stability" option) then waiting for it to stabilise. If you have RCS enabled it will likely waste a lot of mono-propellant if the ship is big and heavy. The best thing to do is disable RCS and let SAS get it stable just using reaction wheels. It may take a while but it will eventually stop rotating.

Then switch to the other ship and dock with it. It won't start rotating again by magic; it will remain still unless you hit it.

If you don't have the patience to wait for SAS to get the ship stable a quick "cheat" is to time accelerate for a moment (hit '.' then ',' straight away). A limitation of the game's physics engine is that rotation is not modeled during time acceleration, and will instantly get set to zero. This is not such a cheat as it appears, since the same thing happens any time the ship is out of physics range (e.g. you go back to the KSC), so it would happen sooner or later anyway.

If you don't already have it I recommend Navyfish's Docking Port Alignment Indicator mod. It makes getting things aligned for docking much easier.

[Reactordrone]

If you can control the ships but you're getting drift you might be better off pointing them Normal/ anti-normal In an equatorial orbit that will be 0° and 180°. They should maintain that orientation in any part of the orbit so then it's just a matter of moving the docking ship into an appropriate position to make the final dock.

[eddiew]

I love your naming convention :>

Your use of the word 'crash' is worrying; it implies the ships aren't stationary relative to each other. If the ships are drifting apart then you need to sort out your relative velocities and get that down to as little as possible at ~100m before trying to dock. There shouldn't be any problem sitting them side-by-side for several minutes, once they've been matched up with gentle shift-key burns. They shouldn't be getting pulled apart by the difference in their orbits at that kind of range. Basic check; you've got your navball in target mode so you can match them up easily with the retrograde marker?

If the lander is fast to manoeuvre via reaction wheels, but out of mono-prop, it's still (in theory) possible to park it in front of the bigger ship's docking port and then edge forward (or move the big ship if it has working RCS), if you are super super gentle on the throttle. Aim across the port as best you can and use SAS to keep you facing retrograde, with navball in target mode. Might take several loops, but eventually you'll get really close. Consider turning the engine's limiter down to 10%, so you have a really fine grained control, and take it slow

[KerBlam]

It's a bit gamey, I know, but you can stop rotation on both ships while you're trying to dock by quickly timewarping and stopping (press > then <).

edit: sorry I didn't notice that this had already been mentioned.

Edited January 13, 2015 by KerBlam

[Starhawk]

I have to echo eddiew above. If your ships are drifting together/apart that means their relative velocities are not matched. Not only is it necessary to get the two craft close together in the orbit, it's also necessary to kill essentially all the relative velocity before you can attempt docking.

The key for me was understanding the markers on the navball. Once you are in target mode {click the velocity readout to change modes}, the yellow markers (prograde/retrograde) show the direction of your relative velocity and the velocity readout at the top of the navball shows the value of the relative velocity. Once you are getting close to your target, you need to bring the relative velocity down to zero and then start the docking process. Bringing relative velocity to zero essentially means you are just matching orbits extremely closely.

Once the two craft are 'floating' next to each other, you can start orienting them the right way.

Hope this helps. Good luck!

[Scarecrow88]

If your ships are still whizzing by each other when they get close, then you haven't matched orbits sufficiently well, as it is the difference in orbits that leads to high closing/parting speeds. When you are close, you need to make sure that your nav ball is in target mode and that you burn retrograde. This has two effects. the first is to kill the relevant velocity between the two ships. The secondary effect is that by doing this, you are also matching the orbits more closely.

[NeroWolfe]

That's got it, thank you one and all.

[eddiew]

Good stuff

[vixr]

The thing that I found to make docking so much easier was getting the prograde marker in the center of my target marker... If the prograde is to the right and above the target I use translation control "I" to get it down and "J" to pull it to the left. This may not be the most efficient way to get it done but it keeps me from burning towards the target to get the prograde to finally move to the center of the target marker... If the prograde marker isn't there, I use the camera to "look" thru the craft at the target. When the engine bell is as straight as I can get it in front of me, I can see where the target is and start translating towards it until my prograde marker shows up... Just a small amount of forward speed toward the target is needed to get the marker lined up, then speed up a little to close the gap...I usually overbuild everything, so the extra RCS waste is within reason for me.